Why cant the White Walkers swim

Warning: There are spoilers ahead if you haven't seen Sunday's "Game of Thrones."

"Game of Thrones" outdid itself Sunday night with a phenomenal showdown that everyone can't stop talking about. Any fans who were previously underwhelmed with season five so far will likely be eating their words after this. 

The episode, titled "Hardhome," featured some great moments with Arya, Daenerys, Tyrion, and Cersei in the first 30 minutes. But the entire last half of the episode focused on Jon Snow and the Wildlings beyond the Wall. 

Jon went to Hardhome with Tormund in order to convince the remaining Wildlings to come south and live in Westeros. Even though the Night's Watch has a longstanding and violent rivalry against the Wildlings, Jon knows the White Walkers are the true enemy. He needs to convince the Wildlings to come with them, or else they will surely be added to the army of the dead. 

In the book series, we only hear about Hardhome secondhand. A man sent there to make peace with the Wildlings sends Jon a letter. It reads: 

At hardhome with six ships. Wild seas...Very bad here. Wildlings eating their own dead. Dead things in the woods...Eight ravens left. Dead things in the water. Send help by land, seas wracked by storms.

Book readers have long wondered what happened at Hardhome, and Sunday's episode was perhaps the most satisfying book divergence yet. As writer David Benioff explained in a feature after the episode aired , "This was an opportunity to go North of the wall and show the audience something that is going to be fresh for everyone. Whether you're a book reader or not a book reader, this is stuff no one has seen."

Jon arrives at Hardhome, and with Tormund helps gather all the leaders of the different Wildling tribes. Though there was some resistance, several thousand of them decide to come with the Night's Watch. Amidst the chaos of all the Wildlings trying to get on boats, there is a sudden avalanche of snow and fog from the cliffs surrounding the fortress.

Almost everyone present realizes something terrible is about to happen, and a Wildling leader quickly orders the gates to be shut — locking thousands of people outside with the coming doom. 

Soon it is clear the White Walkers are here, and they've brought their army. The White Walkers, also referred to as "Others," are capable of reanimating the dead and using them to kill. These zombified people are called wights. 

Here's the first good look we get of them.

They are skeletal, with shining blue eyes and a clear purpose — kill everything in sight.

All hell breaks loose under the attack, as more Wildlings flee to the boats and others stay behind to help keep the wights at bay. Here are some more shots of the attacking corpses.

There are several minutes of bloody sparring and attacks from all sides. For viewers who have been let down by lackluster fight sequences in Dorne this season, this episode completely makes up for it. You can see how the elaborate set and fight choreography were expertly planned. Jon in particular has some spectacular moves.